1. Field of the Invention
This invention relates to a digital map system for displaying three dimensional terrain data. The terrain data is in the form of polygons and is produced from a database of elevation points.
2. Art Background
Systems using elevation data directly have been developed that present an apparent three-dimensional effect as well as some that present a mathematically correct texture-mapped three-dimensional projected display. Both of these systems require a very large amount of storage for terrain data. The latter system also requires specialized hardware. Their high cost has limited their use to military aviation.
The 1992 patent to Beckwith et al. (U.S. Pat. No. 5,140,532) provides a topographical two-dimensional real-time display of the terrain over which the aircraft is passing, and a slope-shading technique incorporated into the system provides to the display an apparent three-dimensional effect similar to that provided by a relief map. This is accomplished by reading compressed terrain data from a cassette tape in a controlled manner based on the instantaneous geographical location of the aircraft as provided by the aircraft navigational computer system, reconstructing the compressed data by suitable processing and writing the reconstructed data into a scene memory with a north-up orientation. A read control circuit then controls the read-out of data from the scene memory with a heading-up orientation to provide a real-time display of the terrain over which the aircraft is passing. A symbol at the center of display position depicts the location of the aircraft with respect to the terrain, permitting the pilot to navigate the aircraft even under conditions of poor visibility. However, the display provided by this system is in the form of a moving map rather than a true perspective display of the terrain as it would appear to the pilot through the window of the aircraft.
The 1987 patent to Beckwith et al. (U.S. Pat. No. 4,660,157) is similar to U.S. Pat. No. 5,140,532. It also reads compressed terrain data from a cassette tape in a controlled manner based on the instantaneous geographical location of the aircraft as provided by the aircraft navigational computer system and reconstructs the compressed data by suitable processing and writing the reconstructed data into a scene memory. However, instead of providing a topographical two-dimensional display of the terrain over which the aircraft is passing and using a slope-shading technique to provide an apparent three-dimensional effect similar to that provided by a relief map as shown in the '532 patent, the '157 patent processes the data to provide a 3D perspective on the display. There are a number of differences between the '157 patent and the present invention, including the following:
1. The '157 patent stores the map as a collection of terrain points with associated altitudes; the large amount of storage required by this approach requires that a tape be prepared for each mission. The present invention uses a reduced number of elevation points representing polygons which results in a reduction of data base storage so that larger geographic areas can be stored so that it is not necessary to generate a data base for each mission. PA1 2. The '157 patent uses a tape cassette for data base storage; the long access time for tape storage makes it necessary to use a relatively large cache memory. The present invention uses a CD-ROM which permits random access to the data so that the requirements for cache storage are reduced. PA1 3. The '157 patent accounts for the aircraft's heading by controlling the way the data is read out from the tape. Different heading angles result in the data being read from a different sequence of addresses. Since addresses exist only at discrete locations, the truncation of address locations causes an unavoidable change in the map shapes as the aircraft changes heading. The present invention stores terrain as polygons which are mathematically rotated as the aircraft changes attitude. The resolution is determined by number of bits used to represent the vertices of the polygons, not the number of storage addresses. PA1 4. The '157 patent accounts for the roll attitude of the aircraft by mathematically rotating the screen data after it is projected. The '157 patent does not show the display being responsive to the pitch angle of the aircraft. In systems such as this the lack of fidelity is apparent to the user. People know what things are supposed to look like and how they are supposed to change perspective when they move. The present invention uses techniques to perform the mathematically correct transformation and projection.
The 1991 patent to Behensky et al. (U.S. Pat. No. 5,005,148) shows a driving simulator for a video game. The road and other terrain are produced by mathematically transforming a three-dimensional polygon data base. The data base does not represent real terrain but rather is created to meet the specific needs of the game. The creation of this database is very labor intensive.
The 1993 patent to Dawson et al. (U.S. Pat. No. 5,179,638) shows a method and apparatus for providing a texture mapped perspective view for digital map systems which includes a geometry engine that receives the elevation posts scanned from the cache memory by the shape address generator. A tiling engine is then used to transform the elevation posts into three-dimensional polygons. One important difference between the '638 patent and the present invention is that the '638 patent stores the map as a large number of terrain points with associated altitudes, thereby requiring a large amount of data storage. The present invention uses a reduced number of elevation points representing polygons which results in a reduction of data base storage.
Also, because the '638 patent uses a large number of texture mapped polygons it requires expensive custom hardware that restricts its use to military applications. The present invention uses a reduced number of elevation points representing polygons and can be implemented with standard commercially available hardware which allows it to be used in more cost sensitive applications such as in general aviation and video games.
The sales brochure from the U.S. Geological Survey shows the availability of Digital Elevation Models for all of the United States and its territories.
The second sales brochure from the U.S. Geological Survey shows the availability of Digital Line Graph Models for all of the United States and its territories. The data includes: political and administrative boundaries; hydrography consisting of all flowing water, standing water, and wetlands; major transportation systems consisting of roads and trails, railroads, pipelines, transmission lines, and airports; and significant manmade structures.
Accordingly, several objects and advantages of my invention are to provide a digital map system that produces a mathematically correct three-dimensional projected view of the terrain while reducing the amount of storage required for the data base and which can be accomplished by using standard commercially available components. The invention can be used in a pilot aid during a flight, in a flight simulator, as part of the control system for a remotely piloted vehicle, or in a video game.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.